One hears a lot about the impedence mismatch between modern object-oriented programming languages used for application development and 40 year-old SQL that is still used for the heavy-lifting in the database. Object/Relational mapping tools were developed to fix the mismatch. One day they will discover that they are trying to fix the mismatch from the wrong end. Until then, we will have to deal with the sometimes huge performance hit imposed by these tools.
Procedural programers love object/relational tools because they can write data manipulation code in the same object-oriented manner as their other application code. They love them so much that they often overlook the performance implications until it is too late and, even after, they have a hard time giving them up. Here's an illustration:
A while back, I got a call from a client in Silicon Valley. Their web application had suddenly had a dramatic downturn in performance. I jumped on a plane. In a very short time I found that there was one statement that was reading about 40 million 8k data pages every time it executed. It took about 12 minutes to run and was paralyzing the server. I took the SQL it was executing and ran it through query analyzer. It read about 10,000 pages and ran in a couple of seconds.
I went to the head of development and the developer himself with this information so they could straighten out the code. It had been written using an O/R mapping tool by an evangelistic java programmer without anyone else's knowledge. It was the only piece of code in a huge application that used o/r mapping and it had brought the application down.
When I am on that client site they put me in the developers email group so I can follow all the conversations. By chance, later on the same day one developer sent an email around mentioning that he had read some things about this cool new O/R tool. The developer who had single-handedly brought down the application with this tool jumped into the conversation announcing that he was already using it and it worked great. He did not say a thing about performance.